1. Field of the Invention
The present invention relates to a path scheduling method and system. More specifically, the invention relates to a path re-scheduling method for rolling mills and a path re-scheduling system for rolling mills, and in particular, to an optimum path schedule determining method for a rolling mill that rolls a coil to be coiled and the like (hereafter collectively referred to "coil"), as well as to an optimum path schedule determining system for such a rolling mill.
2. Description of the Related Art
In a rolling mill having N (N.ltoreq.2) stands for rolling a coil, the determination of a schedule covering an optimum exit thickness of the coil at each stand is important from the standpoint of achieving stable mill operation and maintaining high quality of a finished product.
In a conventional approach to determine an optimum path schedule, a basic path schedule is determined, covering e.g. rolling reductions at respective stands to be distributed as specified in value, and employed for calculation of values of associated parameters at each stand, such as rolling force, bite angle, linear force, neutral point position, torque, power, and rolling speed, and when a calculated value exceeds a specified mechanical limit or conditional limit for stable operation, an optimization is made by changing distribution of rolling forces such as to the offending stand, thereby preparing an optimized path schedule.
With recent advances in production technology and diversifying demands for product quality, however, the actual operation of rolling mills has become extremely complex. For stable mill operation to be still maintained, necessary factors to be considered have increased in number for determination of a path schedule to be optimized yet better, with increased importance to a precise prediction by calculation.
Conventionally employed limits are as follows:
(1) Rolling force. To provide mechanical protection for mill elements such as load cells, a limit is imposed on the withstanding force. Typically, in order to prevent fatigue failures after long periods of operation, a safety factor is multiplied to an actual specified value to be smaller.
(2) Rolling torque. A limit on rolling torque is established so as to protect the drive system elements such as the mill spindle.
(3) Motor power. This limit is established to provide electrical protection for the main motor of the mill.
(4) Bite angle. With hot rolling in particular using a hot strip mill, the bite angle at the end of a coil is a particularly important factor in achieving stable operation. If the rolling reduction of a stand is excessive, so that the bite angle limit is exceeded, the bite at the next stand is adversely affected, thereby risking accidents. This limit is provided to prevent such occurrences.
(5) Unit force per width. In a tandem cold mill that cold rolls a coil, if the unit force per width exceeds a certain value, the condition for lubrication between the coil surface and the roll surface worsens, leading sometimes to surface damages known as heat scratches. Setting this limit is done to prevent such damages.
(6) Neutral point. This limit is also set in a tandem cold mill. If conditions are set so that the neutral point is deviated near the exit or entrance side of the roll bite, or so that it slips out of the roll bite, slipping can occur within the roll bite, this being a direct cause of vibration of the mill. If this slipping is excessive, it can even lead to breakage of the coil, and this limit is set to prevent such problems.
(7) Rolling speed. In order to protect the main motor, it is necessary to check the speed control at each stand of the mill.
It will be understood that checking criteria other than those noted above are generally set, in accordance with running conditions, and that the more limit items there are, the better must be the optimum path schedule.
A conventional method is disclosed, in Japanese Patent Application Laid-Open Publication No. 1-233003, whereby if the predicted power of the motor at a particular stand exceeded a limit value, based on the difference between the predicted motor power and the limit value, an influence factor, which is the calculated amount of power change at other stands for a motor power change that causes a minute variation in entrance and exit thickenesses of coil at each stand, and a standard power distribution ratio are used to distribute the power of the limit-exceeding stand among other stands, so as to correct the exit thicknesses at each stand in the basic path schedule, thereby maintaining the power balance between the stands.
In another method disclosed in Japanese Patent Application Laid-Open Publication No. 5-269514, a number of rolling conditions required for normal operation at each stand are checked and, with regard to a stand at which any limit value is exceeded, based on influence factors of entrance and exit thicknesses of coil for that condition, the basic schedule for that stand is changed so that the limit value is not exceeded.